Laser trimming is used for trimming a thin film resistor (see U.S. Pat. No. 5,081,439 and Japanese Patent No. 2,618,139 for example). Specifically, as shown in FIGS. 10A and 10B, a thin film resistor 102 is formed on a top surface of a silicon substrate 100 of an IC chip. An insulation film 101 is interposed between the thin film resistor 102 and the silicon substrate 100. Also, electrodes 103, 104 are disposed at opposite ends of the thin film resistor 102. Trimming is performed by irradiating the thin film resistor 102 with a laser beam to form a trim cut 105. In the embodiment shown, for instance, an L-shaped trim cut 105 is formed.
It is known that the quality of trim cuts created during laser trimming can vary. For instance, the cutting quality of a linearly polarized laser beam may be adequate in one linear direction, but uncut strips may remain when trimming in another linear direction. Also, trimming at an inadequate laser intensity (i.e., a shortage of the laser beam energy) can cause defects to be created in the resistor. Defects can also be created due to improper focusing of the laser beam and/or due to a relatively small trimming window (i.e., a relatively small width of trimming energy over which a favorable cut can be achieved).
The resistor can be visually examined to detect defects; however, a visual examination may not reveal certain defects. In addition, the resistor can be electrically evaluated for detecting defects. However, an electrical evaluation may not reveal certain defects, especially if the trim cut is relatively long. This is because the electrical characteristics of the resistor are more likely to fall within acceptable ranges as the length of the trim cut increases.
More specifically, as shown in FIG. 11A, a thin film resistor 102 has been trimmed using a linearly polarized laser beam. The trim cut 105 includes a first trim cut 105a, which extends in the Y-direction, and a second trim cut 105b, which extends in the X-direction. As shown, the cutting ability of the linearly polarized laser beam is reduced when trimming in the X-direction as compared to trimming in the Y-direction. In other words, uncut parts remain in the second trim cut 105b, and the uncut parts could be detected with a visual examination. However, uncut parts in the first trim cut 105a, if any, are not obviously present, and such defects would be difficult to detect with a visual examination.
FIG. 11B also shows an example of trimming using a linearly polarized laser beam, in which the trim cut 105 is formed in the Y-direction only. In this case, uncut parts, if any, are not obviously present in the trim cut 105 because it extends in the Y-direction only. Thus, defects in the trim cut 105 of FIG. 11B would be difficult to detect with a visual examination.
FIG. 11C shows an example of trim cuts 105a, 105b disposed in an L-shape and formed using a circularly polarized laser beam. As shown, uncut parts, if any, are not obviously present in the trim cuts 105a, 105b because the circularly polarized laser beam was used. As such, any defects in the trim cuts 105a, 105b would be difficult to detect with a visual examination.